1. Field of the Invention
The present invention relates generally to oscillators, and more specifically to oscillators having amplitude, phase and frequency control.
2. Background Art
Many electronic systems, such as systems for communication or measurements, need stable time references. These time references are often implemented as electronic oscillating circuits, or oscillators. These oscillators generally produce periodic waveforms as an output voltage, current, charge or other electrical variable that can be used as a time reference. In many cases, these output periodic waveforms are sinusoidal, triangular, sawtooth or square waveforms.
Depending on the exact application, the need can exist for a quadrature output signal, a signal that is ±90 degrees out of phase with the first output signal. Moreover, there can be a need to change the amplitude, phase or frequency of the electrical oscillation. Furthermore, a desired feature of oscillators is that they can easily be integrated in integrated circuits in standard processes, consuming as low power as possible to prevent the need for a blower or heat sink on the integrated circuit, or even enabling integration at all.
Referring now to FIG. 1, there is shown one example of a well-known prior art two-integrator oscillator 100. The prior art oscillator comprises two transconductance amplifiers, 102a and 102b, with inputs ui1 and ui2, and output io1 and io2 respectively. The output currents of the two transconductors, 102a and 102b, are fed into capacitors C2 (104a) and C1 (104b) respectively, thus creating two voltage in—voltage out integrators, 106a and 106b. These two integrators each create a 90 degree phase shift. To complete the necessary 360 degrees of phase shift in the oscillation loop, an inversion is implemented between the output of the second integrator and the input of the first. In FIG. 1, this inversion is created by the inverting amplifier 108. In a fully differential version of the prior art oscillator, this inversion can also be created by swapping the positive and negative terminals of one the integrators in the loop.
One problem with such prior art two-integrator oscillators is the signal energy losses inside the oscillation loop 110. Losses may occur, for example, inside the transconductance amplifiers or may be caused by capacitor non-idealities, such as parasitic loss resistances. In order to sustain oscillation, these signal energy losses need to be compensated.
One solution in the prior art for compensating for such signal energy losses inside the oscillation loop 110 is to use transconductance amplifiers. Referring again to FIG. 1, there are shown two undamping transconductance amplifiers gm3 and gm4, 112a and 112b, which are used to compensate for the signal energy losses inside the oscillation loop 110. The two undamping amplifiers, 112a and 112b, are controlled by an amplitude control circuit, 114, that compares the amplitude of the oscillation inside the loop 110 to a desired reference value and controls the two undamping transconductance amplifiers, 112a and 112b, to compensate for the signal energy losses inside the loop. As a result, the oscillation loop 110 will sustain oscillation at the frequency where the loop gain is equal to 1 and the total phase shift in the loop is equal to 360 degrees. In the prior art oscillator of FIG. 1, the oscillation frequency ω0 is given by the equation:
      ω    0    =                              g                      m            ⁢                                                  ⁢            1                          ⁢                  g                      m            ⁢                                                  ⁢            2                                                C          1                ⁢                  C          2                    
The transconductances gm1 (102a) and gm2 (102b) of the two-integrator oscillator may be defined by the equation:gm1=gm2=gm and capacitors C1 (104b) and C2 (104a) are given by:C1=C2=CIn this case, the oscillation frequency ω0 is defined by the equation:
      ω    0    =            g      m        C  
In the prior art architecture depicted in FIG. 1, the frequency and amplitude of the oscillation can be controlled independently. The frequency of the oscillation is determined by the value of the capacitors C1 (104b) and C2 (104a) and the transconductances gm1 (102a) and gm2 (102b), while the amplitude of the oscillation is controlled by the two undamping amplifiers, 112a and 112b, together with the amplitude control circuit 114.
When no amplitude control loop is present, dissipation of the signal energy inside the amplifiers, or in lossy capacitors, causes the initial amplitude of the oscillation to decay exponentially. In such a case, the output signal of the circuit is determined by the initial voltages across the capacitors, 104a and 104b. Referring now to FIG. 2, there is shown the exponential decay of the waveforms for voltages ui1 and ui2 when an initial voltage of 1V is present across C1 (104b) and an initial voltage of 0V is present across C2 (104a).
Another disadvantage of the prior art two-integrator oscillator is the complex circuitry required to control the amplitude, phase and frequency. Such complex solutions can be difficult to design, unstable in operation and expensive to manufacture. Yet another disadvantage of the prior art oscillator in FIG. 1 is that phase control and phase locking is not possible.
Therefore, what is needed is an oscillator in which amplitude, phase and frequency can be easily controlled without the problems in the prior art.